Do you find yourself always setting your bass at -2 or -4 depending on the music? Do you wish you had a flatter bass response from the $1000 Bose system?
If so, this message is for you, but only if you have the A4 sedan -- the Avant is different.

The stock Bose system comes with two 6.5" subs in the rear deck. IMHO those aren't real subs. They seem to be way to sensitive for a true sub and they
seem to have a very narrow (and loud) frequency response in the bass region. This is why I had to constantly change the bass settings depending on
whether or not the music program had its bass in that narrow range or not. I decided to replace the subs with something less sensitive and with flatter
bass response.

I ordered two "butyl surround woofers" from Parts Express (part# 290-350). At less than $20 a piece they're
a steal. They have very good characteristics, sensitivity of 87dB at 1W/1M, wide and flat bass response, and are capable to be mounted as "free air"
woofers. They also have the exact same mounting points as the Bose units, so no fitment is necessary. BTW, I found it interesting that the Bose "sub"
had a very puny magnet. The Parts Express speaker's magnet is three times the size of the Bose unit. I replaced the bose woofers with the parts express
ones. I also purchased a cheap ($3) polyester fiber filled pillow, took it apart and stuffed the polyester fiber between the trunk frame and the rear
deck to reduce the rear deck resonances. I now enjoy a much flatter bass response. I can finally keep both, the bass and treble settings of my Bose
system at 0.

Required tools:

A folding torx screwdriver (The one that has a Swiss Army like arrangement of different torx sizes is perfect for this)

Pliers

Scissors

Step-by-step instructions:

From the trunk, remove the plastic covers that cover the magnets of the bose woofers. The covers are glued to the speakers, so some force is
required to remove them. Don't worry about them though as you can't re-use them due to the replacement speaker's magnet size.

Follow the instructions in the A4.org FAQ to remove the rear deck (hint: use the pliers to squeeze the three plastic snap-in locks).

Remove the torx screws holding the Bose speakers to the plastic cup.

Lift the speaker up, disconnect the wires and remove the speaker.

Take the isolation putty and put it around the bottom edge of the new speaker. The putty must be put in such a way as to sit between the speaker
and the plastic cup when the speaker is inserted into the cup. The putty (which comes with the speaker) has two purposes: it makes the connection between
the speaker and the plastic cup air tight thus ensuring that no sound produced by the front of the speakers leaks to the back of it and it reduces the
transfer of the speaker's resonances onto the cup and the rear deck.

The positive terminal of the Bose's speaker and its associated wire plug are much smaller than the positive terminal of the new speaker. The negative
terminals are the same size. Using scissors, cut the small positive terminal plug away.

Strip the wire and mount the new wider plug (it comes with the speaker) onto the wire using the pliers.

Attach the wires and put the new speaker into the cup.

Do the same for the other speaker.

Turn on the radio and make sure that both speakers work properly. (They will sound funny since your deck is removed.)

If everything is OK, screw back the torx screws and put back your rear deck.

Using scissors tear open the polyester filled pillow

From the trunk, start stuffing the polyester between the trunk frame and the rear deck. You know you stuffed enough when a knock on the rear deck gives
you a nice "dead" resonance free sound.